This invention relates to a method for determining the kerogen content of oil shale. The kerogen analysis of this invention can be performed immediately without requiring laboratory analysis and complex calculations, or destructive techniques.
Vast untapped reserves of shale oil in the form of oil shale exist in this country as well as other locations throughout the world. Increased attention is being devoted to the exploitation of oil shale because of the current need for new energy sources.
The term "oil shale" as used in industry is, in fact, a misnomer, because it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposits interspersed with layers containing an organic polymer called "kerogen" which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the carbonaceous liquid product is called "shale oil."
Oil shale deposits occur in generally horizontal beds, and within a given bed there are an extremely large number of generally horizontal deposition layers containing kerogen known as "varves." The varves typically are nonuniformly dispersed throughout a given bed. In the higher grade oil shales, the varves are commonly cemented together into relatively thick, compact layers. The lower grade oil shales have much thinner varves spaced apart through the bed. For example, a core sample taken from a typical bed may vary from thick, solid oil shale sections to layered sections having the appearance of poker chips. In the core, small layers or nodules of other minerals and rock materials are sometimes found interspersed between the varves containing kerogen. Typical of these other minerals are nahcolite, dawsonite, other salines, dolomite, mudstone, sandstone, tuffs, analcite and bentonite.
Techniques for preparing oil shale for retorting generally comprise explosively expanding a subterranean oil shale formation to form a fragmented, permeable mass of particles containing oil shale. Shale oil then may be recovered from the particles by in situ retorting techniques, or by retorting in surface retorts, for example.
Prior to preparing the oil shale for retorting, the formation is explored to determine the location within the formation containing the highest grades of oil shale. Core samples are taken from the formation and subjected to laboratory analysis to determine the kerogen content of the sample. One such analytical technique is the "Fischer assay" in which a sample customarily weighing 100 grams and representing one foot of core is subjected to controlled laboratory analysis involving grinding the sample into small particles which are placed in a sealed vessel and subjected to heat at a known rate of temperature rise to measure the kerogen content of the core sample. Kerogen content is usually stated in units of "gallons per ton," referring to the number of gallons of shale oil recoverable from a ton of oil shale heated in the same manner as the Fischer analysis.
Such analytical techniques are generally done in laboratories far from the drilling site. This causes a considerable delay before analytical results are available to field personnel conducting the exploration tests. Thus, immediate field decisions on the progress of the exploration program cannot be based on accurate analysis of core samples.
The present invention provides a method for rapidly determining the kerogen content of oil shale. The invention does not require complex calculations or destructive laboratory techniques characteristic of the Fischer assay and other known methods for measuring kerogen content. The invention uses a very small sample which makes it possible to accurately analyze a core sample of oil shale in the field and still have the core sample available for other purposes, such as for corroborating laboratory measurements to be conducted later at a more convenient time. The sample for analysis can be taken along the length of a core, leaving most of the core intact. Thus, field decisions on the progress of the exploration program can be made immediately, rather than waiting for several days, which is a common delay for kerogen assay by Fischer analysis.